1. Field of the Invention
This invention relates to a process for producing water-insoluble water-absorbing resins.
2. Description of the Prior Art
Water-absorbing resins are extensively utilized for physiological commodities, paper diapers or like sanitary materials, drip absorbents, soil water retainers, anti-dew-condensation materials for construction materials and so forth.
A process of producing water-insoluble water-absorbing resins through polymerization of comparatively high density hydrophilic vinyl monomer, an aqueous solution polymerization process and other techniques are well known in the art. Among these techniques there is a process, in which an aqueous solution of hydrophilic vinyl monomer is heat-insulation polymerized in a specific vessel (as disclosed in, for instance, Japanese Patent Application Disclosure No. 108407/1980), a process, in which an aqueous solution of hydrophilic vinyl monomer is polymerized while gels produced under polymerization are pulverized by agitation in a double arm kneeder (as disclosed in, for instance, Japanese Patent Application Disclosure No. 34101/1982), a process, in which a high density aqueous solution of hydrophilic vinyl monomer is polymerized on a belt, the polymerization being carried out simultaneously with drying without applying any external heat (as disclosed in, for instance, Japanese Patent Application Disclosure No. 71907/1983) and a process, in which an aqueous medium mixture containing hydrophilic vinyl monomer and a crosslinking agent is copolymerized by spray or thin film copolymerization in a heat-insulated state (as disclosed in Japanese Patent Application Disclosure No. 147809/1981).
However, when producing water-insoluble water-absorbing resins through polymerization of an aqueous solution containing as high concentration as 30 to 80% by weight of hydrophilic vinylmonomer by either of the polymerization processes noted above, heat of polymerization generated during polymerization causes quick temperature rise to extremely increase the rate of decomposition of the polymerization initiator. In consequence, the weight-average molecular weight (hereinafter referred to as Mw) of the obtained water-absorbing resin polymer is reduced, and also the molecular weight distribution is broadened.
Water-absorbing resins with low Mw and broad molecular weight distribution have problems that water-absorbing performance such as normal pressure water absorption and under-pressure water absorption is reduced. In addition, when a water-absorbing resin greatly containing water-soluble components is used for paper diapers, the stickiness of the diaper surface is increased, or skin reaction or the like will be caused.
Further, when an aqueous solution containing as high concentration as 30 to 80% by weight of hydrophilic vinyl monomer is polymerized by the process disclosed in Japanese Patent Application Disclosure No. 71907/1983 or process disclosed in Japanese Patent Application Disclosure No. 147809/1981, the polymerization initiator is decomposed in a short period of time, resulting in an increase of the residual monomer in the obtained water-absorbing resin polymer. Great residual monomer content have an adverse effect in safety when the resin is used for paper diapers or the like.